Senior Software Development Engineer (Payments)

About the company

ZYSYGY is building Germany's zero-fee payment network. Merchants pay zero transaction fees. We route payments directly over SEPA Instant, bypassing Visa, Mastercard, and the entire card network chain. No hardware. No card reader. Just a phone. We are building the first company to combine a fully software-based merchant POS with a consumer super-app for everyday financial life: payments, transport, utilities, government services, all in one place. Think what UPI did for India. We are doing it for Germany, from the ground up, on banking rails. Role Overview: You will join the core engineering team as a founding engineer. You will own the backend payment engine end to end: transaction flows, wallet management, SEPA Instant settlement, KYB/KYC integration, recurring mandates, refund logic, and the compliance export layer (DATEV, Kassenbuch, GoBD, fiskaltrust). On mobile, you will contribute to the React Native consumer and merchant apps alongside our mobile engineer. You will own what you build from development through deployment, monitoring, and production reliability. The architecture decisions you make in the first six months will run in production for years. What you will do: Design and build the backend payment engine: transaction flows, wallet management, SEPA Instant settlement, offline payment authorization, and recurring mandate billing. Own KYB/KYC integration and the compliance export layer: DATEV, Kassenbuch, GoBD, and fiskaltrust for KassenSichV. Build REST APIs for financial-grade reliability and high-throughput transaction processing, including full R-transaction handling and reconciliation pipelines. Contribute to the React Native consumer and merchant mobile apps alongside our mobile engineer. Own the full lifecycle of what you build, from development through deployment, monitoring, and continuous improvement. Who you are: Meaningful hands-on experience building payment systems, fintech infrastructure, or financial APIs. You have designed a ledger. You understand double-entry bookkeeping, immutable transaction records, and why you never update a financial row. You have solved the partial failure. Money left one wallet and did not arrive in the other. You know how to detect it, resolve it, and make sure it does not happen again. You understand idempotency at a design level. You have built systems that survive client retries, duplicate webhooks, and network drops without double-charging anyone. You have built or reasoned about reconciliation: detecting discrepancies between your internal ledger and an external settlement file, and resolving them reliably at scale. You understand KYC and KYB state machines, what happens when verification status changes mid-transaction, and what a regulator expects from your audit trail. You are comfortable with SQL databases and own the full lifecycle of your code from development through deployment. You can communicate technical decisions clearly to non-technical stakeholders.